Method for coating steel with nickel



United States Patent O 3,479,258 METHOD FOR COATING STEEL WITH NICKEL Thomas Brian Ashcroft, Birmingham, England, assignor to The International Nickel Company, Inc., New York, N .Y., a corporation of Delaware No Drawing. Filed June 27, 1966, Ser. No. 560,893 Claims priority, application Great Britain, July 1, 1965, 27,925/65; Mar. 14, 1966, 11,084/66 Int. Cl. C23f 17/00; C23b 5 0, 5/06 US. Cl. 204-38 11 Claims ABSTRACT OF THE DISCLOSURE Nickel coated steel strip having a sound dense nickel coating is produced by applying to the surface of steel strip an aqueous slurry containing nickel powder, drying the slurry, warm working the dried layer at a temperature between about 200 C. and 650 C., annealing the warm worked strip at about 600 C. to about 700 C. and cold rolling the annealed strip to achieve final densification of the coating.

The present invention is directed to an improved and economical method for producing sound, dense coatings on steel, and more particularly, to a special method for producing nickel coatings of good quality on flat steel products using nickel powder as the source of nickel wherein a major amount of the processing is conducted at relatively low temperature.

In US. Patent No. 3,316,625, granted May 2, 1967, there is disclosed a process for producing a corrosion resistant coating of nickel or a nickel alloy with copper or tin which advantageously contains special amounts of inert particles such as alumina or thoria. The process cornprises applying a layer of an aqueous slurry containing fine nickel powder such as carbonyl nickel powder having a particle size of about 2 to microns, e.g., about 3 to 7 microns, to a prepared steel strip, drying the slurry layer, sintering the dried layer, densifying the resultant layer, and then repeating the sintering and densification steps. The densification is carried out by cold rolling the strip in each case after sintering it and, after the second cold rolling, the strip may advantageously be annealed to enhance its corrosion resistance.

By means of this process, we can prepare coated steel strip which has excellent corrosion resistant properties due to the dense nickel coating. A two stage process, treating each sinter and cold roll as a stage, is required to produce a nickel coating having 100% density, that is o to say, a nickel coating having the theoretical maximum density (and therefore no porosity). It is this 100% density which confers corrosion resistance.

This process as described comprises in effect four steps, namely, sintering, cold rolling, further sintering and further cold rolling. We have now made the surprising discovery that, to achieve 100% density of the coating, it is not necessary to sinter the dried slurry layer.

It is an object of the present invention to provide a method for coating fiat steel strip with nickel in a particularly economical way while providing a dense nickel coating on the steel.

It is a further object of the invention to provide a method for nickelcoating flat steel strip wherein a minimum of elevated temperature heating steps are required.

According to this invention, a layer of aqueous slurry containing nickel powder is applied to the strip, the slurry layer is dried, the dried layer is compacted by rolling the strip at an elevated temperature but below the sintering temperature of the layer, the coated strip is annealed and then subjected to cold rolling to achieve the final densification of the coating. The special step of compacting the dried layer by rolling at a temperature below the sintering temperature may conveniently be called warm compaction.

A two stage process is still necessary to achieve 100% density but three of the steps involved in these stages have now effectively been replaced by two steps, those of Warm compaction and annealing.

Generally speaking, sintering of fine nickel powder, such as carbonyl nickel powder, occurs at temperatures of about 700 C. and above, so that the warm compaction is carried out at temperatures below this. The actual temperature used may be quite low, say no more than 200 C., but is preferably somewhat higher than this. Above 500 C., however, oxidation will occur in air and at this temperature or above the warm compaction must be eflected in a protective or reducing atmosphere. A suitable range of temperature for the warm compaction in which the operation may be conducted in air is from 200 C. to 450 C.

It is found that if a high enough pressure is applied during the warm compaction and the temperature is high enough, the powder particles, e.g., carbonyl nickel powder having a particle size of about 3 to 7 microns, will bond together and to the strip, though if the pressure is too high, the coating will crack even though it sticks to the strip. The pressure required depends on the thickness of the strip and the size of the rolls, varying substantially in accordance with the square root of both the strip thickness and the roll diameter, and can easily be determined by test.

Coated steel strip prepared according to this invention has good corrosion resistant properties. It is, however, found that these properties may be improved still further if the strip bearing the dried slurry layer is heated to a temperature in the range 500 C. to 650 C., that is to say, as high as possible but below the sintering temperature of the dried layer before the warm compaction step.

The cold rollin of the coated strip may be carried out if desired in two or more stages, the coated strip being annealed between each cold rolling. In a first stage, for example, the coating may be compacted to to 85% of the theoretical maximum density, and in a second stage to the maximum density, that is, all porosity is eliminated.

The step of cold rolling inevitably introduces stresses into the surface coating of the strip, and it is well known that a stressed material is more prone to corrosion attack than an unstressed one. It is therefore preferred to sub ject the coated steel strip to a final annealing step to relieve any stresses in the coating and thereby further enhance the corrosion resistance of the coated strip.

The composition of the slurry and the ways in which it may be applied to the strip may be the same as is set forth in US. Patent No. 3,316,625. Thus, the aqueous slurry may contain about 45% to about by weight, of fine nickel powder having a particle size of about 2 to about 10 microns and up to about 10% by volume of inert particles such as alumina or thoria having a particle size of about 0.01 micron to about 50 microns. The slurry may contain about 0.1% to about 5%, by weight, of a heat-volatilizable organic binder such as methyl cellulose and a wetting agent. As before, the slurry must first be dried at low temperature and it is important not to boil it during the drying. Most preferably, the slurry will contain a binder, not only to bind the nickel powder in the layer of slurry formed on the strip but also to aid in suspending the powder in the slurry, and this binder may suitably be a methyl cellulose. If methyl cellulose is present in the slurry, then the dried layer of slurry on the strip must be heated to drive it off. This may be done in the course of warming the strip prior to the warm compaction step, a temperature of from 250 C. to 450 C.

in a reducing atmosphere being suitable. Heating the dried layer to 500 C. to 650 C., as in the preferred embodiment of the invention, also insures complete removal of the methyl cellulose and, in addition, reduces the residual carbon content of the layer. It appears that this purification of the nickel layer insures that the coating on the steel strip has a superior corrosion resistance to that obtained hitherto.

Annealing of the coated strip, whether as the essential step of the process before cold rolling or as the optional additional step after cold rolling, is preferably carried out in the range 600 C. to 700 C., that is to say, below the temperature at which significant sintering will occur. Annealing may be carried out at higher temperatures, for example, up to 800 C., but then sintering will also occur, and the economic advantages of working at the lower temperatures will be lost.

Four examples will now be given; Examples I, II and III illustrate the invention in its widest terms, and Example IV illustrates the preferred embodiment of the invention in which the strip bearing the dried slurry layer is subjected to a temperature in the range 500 C. to 650 C. before warm compaction.

EXAMPLE I A creamy slurry having the following composition was formed:

Water milliliters 100 Carbonyl Nickel powder grams 275 Methyl cellulose do 0.75 Supronic B75 milliliter 0.75

Alkoxylated polyethylene glycol surfactant.

The bulk density of the nickel powder was 1.8 grams per cubic centimeter and the particle size from 3 to 4 microns.

The slurry was applied to a degreased cold rolled annealed mild steel strip 3 inches wide and 0.062 inch thick to form a layer approximately 0.009 inch thick. The coated strip was dried at about 60 C. in air, heated for five minutes to a temperature of 380 C. in a reducing atmosphere of cracked ammonia and then warm compacted by rollingin air between rolls adjacent to the furnace. It is found that there is a temperature drop from C. to C. during the transfer from the furnace to the rolls, so the actual rolling temperature was between 360 C. and 370 C. The roll diameter was 8 inches and the load applied to the rolls was approximately 8.5 tons per inch width of the strip.

The compacted strip was then annealed for one hour at 700 C. in cracked ammonia, allowed to cool and then cold rolled to 0.030 inch thick. The final thickness of the coating on the strip, which had a good finish, was 0.0015 inch thick. A sample of the coated strip was immersed in boiling water for six hours; the coating was substantially impermeable, showing negligible porosity.

EXAMPLE II A creamy slurry was prepared having the following composition, the methyl cellulose, nickel powder and surfactant being the same as those used in Example I.

Water rnilliliters 200 Methyl cellulose grams 1.5

Carbonyl Nickel powder do 525 Supronic B 75 milliliters 1.5

Silicone Antifoam Emulsion RD do 1.5 Ammonium Hydroxide (specific gravity 0.0880) The slurry was sprayed onto three samples of cold rolled annealed steel strip 3 inches wide and 0.062 inch thick to give a layer 0.012 to 0.013 inch thick. The strips were then dried in air, heated to 381 C. to 385 C. in cracked ammonia, and warm compacted by rolling in air between rolls adjacent to the furnace as in Example I. The rolling loads were between 7.07 and 7.66 tons per inch width of the strip.

The compacted strips were then annealed for 1 hour at 700 C. in cracked ammonia, allowed to cool, and cold rolled to 50% of their thickness using white spirit as lubricant. The strips were electroplated in conventional manner with 10 microinches of chromium, and subjected to the copper accelerated Acetic Acid Salt Spray Test (see British Standard 1224:1965, Appendix F). After 16 hours exposure, specimens of all three strips had protection ratings of 9, expressed according to the ratings given by Bigge in Plating, Volume 47, part 2, page 1263.

EXAMPLE III A creamy slurry was made up from the following materials:

Carbonyl Nickel powder grams 475 Water milliliters 200 Methyl cellulose (sold under the trademark Celacol M450) gr s 2 Supronic B 75 milliliters 1.5

The nickel powder had a bulk density of 1.8 grams per cubic centimeter and an average particle size of 3 microns to 4 microns.

This slurry was applied by the curtain coating technique, this is to say, by allowing the slurry to run over a weir onto the strip below, to hot rolled steel strip four inches wide and 0.056 inch thick previously pickled in 10% HNO to remove any scale. The thickness of the layer of slurry was 0.014 to 0.015 inch.

The strip was dried in air, heated to 419 C. in cracked ammonia, transferred to rolls adjacent to the furnace as in Example I, and warm compacted in air. The rolling load was about 9.2 tons per inch widh of the strip. Subsequently, the strip was annealed for 1 hour at 700 C. in cracked ammonia, allowed to cool, and then reduced in thickness by 50% by cold rolling using white spirit as lubricant. The strip was electroplated in conventional manner with 10 microinches of chromium, and subjected to the CASS Test as in Example II. After 16 hours exposure, the sample of strip tested had a protection rating of 9.

EXAMPLE IV A creamy slurry having the following composition was made, the nickel powder being the same as that used in Example I:

Water milliliters Methyl cellulose powder (solid under the trade mark Celacol M20) gram-.. 1 Carbonyl Nickel powder grams 237.5 Supronic B 75 milliliter 0.75

The slurry was applied to two samples of cold rolled and annealed steel strip 3 inches wide and 0.062 inch thick which had previously been degreased, pickled in 10% HNO for 30 seconds and then rinsed. The object of pickling the strip was to roughen its surface, and, hence, to increase the adhesion of the nickel coating to be formed on it. After drying in air at about 60 C., the thickness of the resulting layer was 0.017 to 0.019 inch.

The samples were then heated in cracked ammonia to a temperature of 600 -5 C., over a period of 10 minutes and then allowed to cool to 410 C., transferred ,to a rolling mill with 8 inch diameter rolls and compacted in air under rolling loads of 8.8 and 10.1 tons per inch, respectively, at a rate of 10 feet per minute. The compacted strips were then annealed for 1 hour at 700 C. in cracked ammonia, and cold rolled with the use of white spirit as a lubricant to reduce their thickness by 50%.

Both samples of strip were then electroplated in conventional manner with 10 microinches of chromium, and subjected to the Acetic Salt Test (see British Standard 1224:1965, Appendix H). The protection rating of both of the samples, expressed according to the ratings of Bigge, was 9 after 120 hours.

The invention may also be used for the production of coatings of nickel and copper, or nickel and tin, by altering the composition of the slurry to include copper or tin powders therein.

Inert electrically non-conductive particles such as alumina or thoria may be included in the initial nickel-containing slurry and chromium deposited on nickel coatings thus produced is of the micro-cracked type. The initial steel strip may be coated with copper or tin as by an electrolytic process and adherence of the nickel coating to the steel is thereby facilitated.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

I claim:

1. A method of forming a corrosion resistant coating on steel strip comprising applying a layer of an aqueous slurry containing fine nickel powder to the strip, drying the slurry layer, compacting the dried layer by warm rolling the strip at an elevated temperature between about 200 C. and 650 C,, annealing the coated strip and densifying the coating by cold rolling.

2. A method according to claim 1 in which the strip bearing the dried slurry layer is heated to a temperature in the range 500 C. to 650 C. before the strip is rolled.

3. A method according to claims 1 or 2 in which the compaction of the dried slurry layer is carried out at 200 C. to 450 C.

4. A method according to claim 1 in which the coating is densified by cold rolling in at least two stages, the coated strip being annealed at temperatures of about 600 C. to about 700 C. between each cold rolling.

5. A method according to claim 1 in which the coated strip is annealed after the final cold rolling.

6. A method according to claim 1 in which the aqueous slurry contains a binder.

7. A method according to claim 6 in which the binder is a methyl cellulose.

8. A method according to claim 1 in which inert electrically nonconducting particles are introduced into the coating as a dispersed phase, and a chromium coating is subsequently applied electrolytically to the coating.

' 9. A method according to claim 8 in which the inert particles are of thoria or alumina in the size range of 0.01 to microns.

10. A method according to claim 1 in which the steel grip is coated with a layer of metal from the group con sisting of copper and tin before the application of the layer of slurry.

11. A method according to claim 1 in which the nickel containing slurry also contains copper powder up to 10% by weight of the nickel plus copper content.

References Cited UNITED STATES PATENTS JOHN H. MACK, Primary Examiner W. B. VAN SISE, Assistant Examiner U.S. Cl. X.R.

Flint 29420.5

"P1050 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,479,258 Dated November I8. 1969 Inventofla) Thomas Brian Ashcroft It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 3, line 68, change "0.0880" to --0.880-.

Col. 6, line 15, (line 2 of claim 10) change "grip" to --strip.

Signed and sealed this 25th day of May 1971.

(SEAL) Attest:

EDWARD M.FLETCHER, JR. WILLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents 

